The motor to which this valve device is applied serves for example to ensure rotation of the turret of a machine such as a hydraulic shovel, or to ensure translation of tracked or wheeled machines having a considerable mass.
This may be a hydraulic motor of the so-called "fast motor" type (1000 to 2000 rpm) driving a reduction gear, or a so-called "slow motor" (whose speed of rotation is for example of the order of 100 rpm, for example of the type incorporating radial pistons.
In operation, a circulation of fluid is maintained in the motor and one of the principal ducts is placed under pressure in order to perform the role of supply duct, while the other of these ducts is under relative depression and is connected to a fluid evacuation in order to perform the role of exhaust duct.
From an operational situation at a given drive velocity, stop of the motor is obtained by effecting a phase of deceleration, then by obturating the supply and exhaust ducts. During the phase of deceleration, the pressure in the supply duct becomes the low pressure, while the pressure in the exhaust duct becomes the high pressure. Finally, when the principal ducts of the motor are obturated, i.e. when this motor is isolated, the fluid located in the exhaust duct is at a pressure higher than that of the fluid located in the supply duct. This phenomenon is further reinforceed by the fact that, due to its high inertia, the driven mass tends to continue its initial movement.
On flat ground, equilibrium of the system is attained only when the pressures in the supply and exhaust ducts are substantially equal. On sloping ground or when the driven mass is in inclined position, equilibrium of the system is attained when the difference between the pressures in the supply duct and in the exhaust duct attains a given value (positive or negative) which makes it possible to compensate for the slope in order to maintain the mass immobile.
In any case, in order that the motor and the driven mass be effectively stopped in a stable position, the difference in pressure between the supply and exhaust ducts must attain a given value: zero, positive or negative.
It has been indicated hereinbefore that, when the supply and exhaust ducts are obturated, the exhaust duct is under excess pressure, which excess pressure is further increased by the inertia of the driven mass. Such excess pressure tends to push the driven mass in a return movement in the opposite direction, which amounts to tipping towards the supply duct which is obturated, the excess pressure of the exhaust duct which is also obturated.
Furthermore, the hydraulic fluid is slightly compressible. Consequently, after isolation of the motor, the inertia mass continues its movement until the pressure in the exhaust duct attains a maximum value corresponding to the compression of the fluid present in this duct. The effect of the return movement of the mass will be to increase the pressure in the supply duct until the fluid present in this duct is taken to a pressure of compression substantially equal to the maximum pressure which prevailed in the exhaust duct just before this return phase is started.
Of course, this return phase is followed by a new phase of displacement in the initial direction, during which there is produced a pressure reduction in the supply duct and a compression in the exhaust duct.
In this way, after obturation of the supply and exhaust ducts, the driven mass is animated by an oscillating movement of which the frequency, for turrets of machines such as hydraulic shovels, is of the order of 1 Hz. Although this oscillating movement has a low relative amplitude and it is finally braked naturally due to the phenomena of friction, it is obviously extremely inconvenient, particularly when it is question of placing the mass driven by the motor in a very precise position by stopping the motor without mechanical braking.
Paradoxically, this phenomenon of oscillating movement was less of a nuisance when the drives were effected with the aid of motors of lower performance, in which relatively considerable leakages limited the compression in the supply and exhaust ducts. The motors have been gradually improved, in particular to improve efficiency thereof, in order to reduce the duration of the phases of acceleration and to facilitate handlings under difficult conditions, for example in an inclined position.
In order to limit the oscillations, i.e. to reduce the amplitude thereof and finally to stop them, it is known to use a damping device consisting in creating, between the supply and exhaust ducts, leakages which supply a transfer volume. Further to the isolation of the motor, the difference in pressure between the supply and exhaust ducts may be at least partially compensated by the fluid available in this transfer volume.
Another system consists in permanently allowing leakages between the supply and exhaust ducts of the motor.
These systems are not entirely satisfactory insofar as, on the one hand, they amount to reducing the efficiency of the motor which, furthermore, it is sought to increase by improving this motor, and as they render a precise positioning of the driven mass when the motor has stopped, virtually impossible. In fact, for example when the motor serves to drive the turret of a hydraulic shovel, the effective stop of the turret will be effected with an angular deviation corresponding to the circulation of the fluid available in the transfer volume, with respect to the target angular position in which isolation of the motor has been controlled.
It is an object of the present invention to overcome the drawbacks set forth hereinabove by proposing a simple, reliable device which makes it possible to brake and cancel the oscillations of the system after isolation of the motor very rapidly, whatever the conditions of drive of the mass, in particular whether they be driven on sloping land or in inclined position, or on flat land.